One field of application for the invention is holding bones in a relative position, for example to aid in the healing of breaks or the positioning of bones in the treatment of spinal deformities or spinal degenerative diseases or trauma diseases, or otherwise to correct abnormal curvatures of the spine. Other bone deficiencies and abnormalities may also benefit from embodiments of the present invention.
The spine is formed of superposed vertebrae, normally aligned along a vertebral axis, from the lumbar vertebrae to the cervical vertebrae, each having a posterior wall from which projects a spinous process and two lateral edges from the walls of which there project ribs and/or transverse processes and/or lamina. If the spine of a person has abnormal curvature, the vertebrae are typically inclined relative to one another and relative to said vertebral axis.
In order to straighten the vertebral column as a remedy for this situation, the lateral edges of the vertebrae on the concave side can be moved away from one another and supported at distances from one another substantially equivalent to the distances between the lateral edges on the other side. Devices known in the art to hold the vertebrae relative to one another include screws that are inserted into the vertebrae or hooks that are inserted along the internal wall of the spinal canal and rods adapted to connect the screws or hooks.
When using a hook and rod system, pairs of hooks are generally inserted into each vertebra, one on each side, near the pedicle. The hooks typically have heads that project from the posterior wall of the vertebra, one on each side of the spinous process. The heads can be tulip-shaped and adapted to receive a rod that is immobilized by a nut screwed onto the head and contacting the rod. The heads of the hooks situated on either side of the spinous process can then be connected together and fixed in position by two rods approximately parallel to one another and to the axis of the spine. However, using such hooks can be difficult because their use increases the risk that the physician (or other operative) might contact and potentially damage the spinal cord that extends along the centre of the spinal canal (which can result in paralysis of the patient).
Using a screw and rod system reduces this risk, but has other drawbacks. The screws typically have tulip-shaped heads and are inserted in pairs into the pedicles on each side of the spinous process on the posterior wall of the vertebrae. The screws therefore constitute fixing points on the vertebrae for holding the vertebrae in a fixed position relative to one another. However, the screws are inserted into the pedicles of the vertebrae, which in some cases are small or are deteriorated and can be damaged or do not provide sufficient purchase to permanently hold the screw.
To solve this problem, the PCT applications WO 2004/010881 and WO 2007/036657 in the name of the applicant relate to vertebral fixing devices to fix an end of the connecting member, for example, a rigid rod, to a vertebra. These devices comprise a connecting part or head which can be mechanically fixed to one end of the rod and a conformable elongate ligature for connecting the head to at least one rib and/or transverse process and/or lamina, the head maintaining a traction applied to the ligature.
As explained in the above applications, each vertebral fixing device produces a rigid connection between the vertebra and the head of the fixing connecting device and, consequently, between the rod and the vertebra. However, in some situations it is desirable to allow a controlled relative mobility between two or more adjacent vertebrae.
To solve this problem, PCT applications WO 2002/07622 and WO 2002/07621 in the name of the applicant relate to flexible linking pieces for stabilizing the spine.
The linking piece has the general shape of a rod having two rigid ends, for cooperating with fixing devices secured to two adjacent vertebrae and an intermediate part which is made with an elastically deformable material.
Due to the elasticity of the intermediate part of the rod, a relative mobility exists between the two adjacent vertebrae.
However, the manufacturing of such elastically deformable linking members is rather expensive and the type of mobility allowed by such linking members is not adapted to all situations.